I want to thin out some waterproof silicone sealant (the stuff that comes in tubes for use with a sealant gun and smells strongly of acetic acid when wet) - so that it can be applied with a brush - I tried googling around, but I can’t seem to find out what solvent, if any, would be suitable.
I understand these sealants typically cure through exposure to atmospheric moisture, so I guess that rules out water…
Any ideas? Or is what I’m asking impossible?
I have no advice, and while I guess you might get a response here, I’d try a forum for woodworking, homebuilding or boat building. Lots of knowledgable people when it comes to finishing, gluing, sealing, etc., at woodcentral.com and taunton.com. Both of these sites tend to deal with furniture building and the like, but there are people with experience in all types of construction, including boat building, kayak construction, etc. The Taunton site in particular has a Finishing forum in the ‘Knots’ section (you’ll have to search around for a while and register to pose a question), and there are a crew of people in there who just live for questions like yours.
As far as a forum for boat building, you’d know more about that than most of us. 
Ultimately, however, I have a feeling you’re probably just not supposed to thin out waterproof sealant that comes out of a gun. I’ve thinned out lots of stuff using either turpentine, denatured alcohol (methyl hydrate) or mineral spirits, but I’m betting none of these will do work to thin out a waterproof sealant.
According to this article, naphtha will do the trick, so you can use lighter fluid as your additive. Here’s the Wikipedia article on Naphtha if you’re looking for a source other than lighter fluid. The first article mentions the existence of (but doesn’t name) pre-thinned silicone adhesives. The author also cautions that thinning the solvent will result in shrinkage (and therefore strain) on your bonded parts:
Thanks for that - the shrinkage could actually be a big plus - I’m looking at the idea of building a very lightweight boat covered with waterproofed canvas - the shrinkage might actually help to pull the skin taut and stiffen the whole structure.
I think a nonpolar solvent like hexane would be fine. The silicone rubber in a tube, sometimes called RTV for “room temperature vulcanization”, is polydimethylsiloxane, which is nonpolar and ought to be miscible with other things that are.
You might research how fabric-skin aircraft are constructed. Seems to me they use butyrate dope. Silicone rubber would be pretty expensive and wouldn’t add much tensile stress to the fabric because its modulus is very, very low. The silicone rubber would, however, be very waterproof.
Any particular reason to reject fiberglass resin? You’ll have to do it in batches, as the product is catylized, but it ought to work.
It’s either expensive (epoxy) or unpleasant to work with (polyester) - I’m also not sure it has the flexibility I’m looking for. I’m still in the thinking-through stages of this project, but there’s a possibility that I’ll build it to be collapsible (of course I’ll also have to pre-shrink it this way) - in which case the skin will have to be waterproof and extremely flexible/foldable - I don’t think two-part resins will do that.
I’m not sure if this would work in your case, but I’ll throw it out there for your consideration. I’ve done some work in Utah and Colorado’s Morrison formation for a Doctor in Paleontology, a museum director. To deeply penetrate, preserve and not harm the dino bones we’d excavated, we used SuperGlue diluted with acetone. While our mixture was probably far more dilute than what you’d be working with, it did penetrate well, evaporated nicely and wasn’t overly noxious to work with.
Low-odor Mineral Spirits is what you’ll want to use. I use a 1:3 ratio of GE silicone II to mineral spirits for seam-sealing tents and it is very effective. Pretty much everyone I know uses this mixture to weather-proof silnylon tents and tarps.
In your case I would maybe use slightly less mineral spirits, for a more “gooey” consistency.
That sounds promising - I think that’s what is sold as ‘white spirit’ here in the UK.
I have an option for you in lieu of trying to thin out the silicone coming from a cartridge or sausage. GE Momentive makes a 100% Silicone coating, SEC2500 SilShield that is spray, roller or brush applied. UV stable and impervious to water. Does not need to be heated to be applied and can be layed to whatever thickness you desire. Self healing if fasteners need to be used after application.
When it comes to skin on frame boat building, I’m not aware of any better online source than the greenland kayak forums:
http://www.qajaqusa.org/forums.shtml
I’ve never heard of anyone using silicone for coating, with canvas skins the traditional coatings are oil based house paint, and mixtures of boiled linseed oil and beeswax. More popular these days is nylon skins with Hypalon or two-part polyurathane coatings. On mine I used pre-coated polyester fabric as described here:
Aircraft skin is might thinner and more delicate than is usually used for boats, but I have heard of it being done.
zombie or no
** Mangetout ** did you finish your project?
Hi i am also looking for silicone thinner and followed up on your suggestion.
Where did you find GE SEC2500 SilShield* AWB. I mean actually purchased some.
I am trying to go through GE… but thought you might have a quicker route…Need several gallons
Thanks
John B
Georgetown tx
matchbook69@gmail.com
Silicone is pretty soft and abrasion would chew it up. I’m not sure it would be durable enough for use on a boat exterior fabric for extended use.
I had some “odorless” mineral spirits… all I can say “NOPE”… it’s white gooey stuff, but definitely not the mineral spirits I remember. I read somewhere it’s vegetable based.
What DID WORK (for me)… is charcoal lighter fluid… Kingsford Odorless… works like a charm!
I needed my sealant to remain transparent, this did the trick!
It doesn’t have an ingredients list… (sic) but I figured it was similar to lighter fluid, which others have stated works… slow burning, minimal residue… only in a charcoal lighter fluid, probably even more safe since you cook over its remains.
I have seen video on youtube making mold using Silicone cauck where a guy uses Napth, other guy uses Industrial Maintenance coating thinner & the brushes that diluted silicone on pattern to make mold & at the end it dries rubbery, So i think most of organic stuff may be working as diluent for silicone, You can give try to various solvents as diluents
You can also use one of the Devcons. They are flexible rubber coatings out of a can and come in various durometers when cured. I have used them to coat cloth. And of course, Phil Swift’s Flex Seal products.
I’ve done a lot of work with Silicone, and have often needed to thin it for various applications. As another reply stated, you can use Mineral Spirits (known as “white spirits” in many countries) typically found at any hardware store, Walmart, etc.
Most people don’t need a huge amount of silicone for their projects, and just buy a tube (or as many tubes needed) of 100% SILICONE CAULK, that’s most commonly thought of as “caulk for cracks and gaps” (like around a window, shower, tubs, sinks, etc). – 
MAKE SURE IT’S CLEAR SILICONE, AND IS CLEARLY MARKED “100% SILICONE”, and does NOT have anything like “latex”, “UV protection additive”, etc. Just PLAIN SILICONE!
I find it easiest to use the cheap caulk gun sold where caulk is, and squeeze out what I expect to use that day (it’s not fast drying, and you easily have a 12 hour working window) into a cheap glass bowl, glass jar (jelly, pickle, mayo jars with lids are GREAT, as you can mix up your silicone “paint”/solution and sealed with a top on the jar, it won’t cure and remains usable for months!), Or old Tupperware I’ve bought for nothing at Goodwill or dollar store. DON’T MIX IN PLASTIC OR FOAM CUPS/BOWLS!
I wear inexpensive plastic gloves I buy at the grocery store, NOT the medical latex gloves, but the cheap “food service gloves” that cafeteria workers and restaurant workers use! They’re about a nickel a pair (much cheaper than medical latex gloves!) and work fine for working with silicone.
Once I’ve squeezed out the needed silicone into my bowl/jar, I add a small amount of Mineral Spirits (MS)…(about a teaspoon of MS for a cup/8 oz of silicone)…to begin thinning the silicone. Don’t add a lot of MS to start! It sloshes around and creates a mess. Start with a little, and just add more MS until you reach the desired consistency!!
Straight from the caulk tube, silicone will be about the thickness of very cold ice cream, straight from the freezer. For most waterproofing applications, I thin the SILICONE to a consistency of pancake syrup. I don’t want it watery, but also NOT “gloopy” and thick like mayonnaise. Too thinned out, and you’ll basically have plain Mineral Spirits and very little SILICONE, and require so many reapplications and dry time you’ll regret over thinning!
You can mix with just about whatever you’ve got laying around. I’ve used the free chopsticks my grocery store offers, next to their sushi area (I pickup a handful of chopsticks every so often), disposable spoons, or anything else I don’t care about or plan to keep. DO NOT use your wife’s, mother’s, partner’s kitchen utensils! You’ll not only ruin the utensils, but you’ll likely bring unwanted attention to your projects and yourself!
Once you’ve got your silicone thinned to the consistency you prefer for your project, I find it easiest to apply it with hog hair “chip brushes” or the cheapest paint brush I can buy. Remember, after you’ve used the brush, the silicone WILL DRY on the brush hairs, and render it useless for anything. And you don’t usually need to worry about brush strokes as the mixture is thin enough to self level when painted onto a surface. If you do need to apply smoothly - like you’re using a thicker mix - you can use cheap foam brushes or rollers.
** I often use some form of “colorant” when mixing up a silicone waterproofing mixture. It helps to identify areas that haven’t been treated. Water based colorants don’t mix easily with silicone (like food coloring), but you can force it to blend usually… just don’t use much. I usually just add a drop or two of acrylic paint (small student grade artist bottles of any color are like $1 at Walmart or at supply store, and last forever) that contrasts slightly with whatever I’m coating.
I’ve also found Bic pens, or any cheap ballpoint pen, are great for coloring silicone! Just pull apart the pen, and using scissors, clip off a small section of the ink tube and let it fall into your silicone/Mineral Spirits mixture, when you’re thinning/mixing your batch. The ink inside the small retaining tube will naturally ooze out and dissolve into the mixture, as the Mineral Spirits thins it easily. YOU DON’T NEED MUCH INK! A rice sized section of a ballpoint pen ink tube clipping will color your Silicone enough to help you identity where you’ve applied the mixture, how thick a layer had been applied, and helps identify thin areas, as well as help you notice areas that get damaged in the future!
As for the OP’s question about using this waterproofing method on fabric for a boat, I think it’s a brilliant idea! I’ve used this DIY waterproofing on all types of fabrics, and have been surprised at the stability, durability, and longevity of the coating!
I’ve discovered that the more “loose knit” or “thin weaved” fabrics…like muslin, cheap bedsheet fabric, absorb the waterproofing mixture THE BEST… Whereas canvas, denim, and thicker weaved cloth, it absorbs less and more “coats” or “sits on top” of the fabric. With the thinner weaved fabrics, you can use a thin application if that’s all you want, and the fabric remains almost unchanged in flexibility, color, feel, etc. Or, you can use a thicker application, or multiple built up applications, and create an almost “rubber fabric” that’s similar to how fiberglass is impregnated into resin… The thin fabric becomes the “scaffolding” that’s supported by the silicone mixture. It’s very flexible, and highly resistant to abrasions.
Hope this gives you some ideas and possible solution to your project. I think you thought up a unique idea for your challenge!
Hi,
I found this thread while searching methods of thinning out RTV.
I have a lot of amature experience with boat building and no, hard resins like epoxy or polyester (yuck!) are not appropriate unless applied to an already rigid structure, a mold, an existing hard part, etc.
I think your idea of using RTV , thinned, on a fabric for water proofing and shrinkage is a great idea.
I am a fabric covered aircraft owner, and we do use dope for that, but the air is 800 times less viscous than the water. So the doped fabric would be too rigid and would crack just as the fabric spans away from the Ridgid structure members. RTV would make an excellent replacement for any of the harder resins, epoxy, dope, or polyester resin.
Your idea is spot on.
I want to thin RTV to use it as a waterproof paint for circuit boards, an economical substitute to conformal coating.
Cheers!
jg